Documentation/admin-guide/pm/cpuidle.rst

.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

.. |struct cpuidle_state| replace:: :c:type:`struct cpuidle_state <cpuidle_state>`
.. |cpufreq| replace:: :doc:`CPU Performance Scaling <cpufreq>`

========================
CPU Idle Time Management
========================

:Copyright: |copy| 2018 Intel Corporation

:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>


Concepts
========

Modern processors are generally able to enter states in which the execution of
a program is suspended and instructions belonging to it are not fetched from
memory or executed.  Those states are the *idle* states of the processor.

Since part of the processor hardware is not used in idle states, entering them
generally allows power drawn by the processor to be reduced and, in consequence,
it is an opportunity to save energy.

CPU idle time management is an energy-efficiency feature concerned about using
the idle states of processors for this purpose.

Logical CPUs
------------

CPU idle time management operates on CPUs as seen by the *CPU scheduler* (that
is the part of the kernel responsible for the distribution of computational
work in the system).  In its view, CPUs are *logical* units.  That is, they need
not be separate physical entities and may just be interfaces appearing to
software as individual single-core processors.  In other words, a CPU is an
entity which appears to be fetching instructions that belong to one sequence
(program) from memory and executing them, but it need not work this way
physically.  Generally, three different cases can be consider here.

First, if the whole processor can only follow one sequence of instructions (one
program) at a time, it is a CPU.  In that case, if the hardware is asked to
enter an idle state, that applies to the processor as a whole.

Second, if the processor is multi-core, each core in it is able to follow at
least one program at a time.  The cores need not be entirely independent of each
other (for example, they may share caches), but still most of the time they
work physically in parallel with each other, so if each of them executes only
one program, those programs run mostly independently of each other at the same
time.  The entire cores are CPUs in that case and if the hardware is asked to
enter an idle state, that applies to the core that asked for it in the first
place, but it also may apply to a larger unit (say a "package" or a "cluster")
that the core belongs to (in fact, it may apply to an entire hierarchy of larger
units containing the core).  Namely, if all of the cores in the larger unit
except for one have been put into idle states at the "core level" and the
remaining core asks the processor to enter an idle state, that may trigger it
to put the whole larger unit into an idle state which also will affect the
other cores in that unit.

Finally, each core in a multi-core processor may be able to follow more than one
program in the same time frame (that is, each core may be able to fetch
instructions from multiple locations in memory and execute them in the same time
frame, but not necessarily entirely in parallel with each other).  In that case
the cores present themselves to software as "bundles" each consisting of
multiple individual single-core "processors", referred to as *hardware threads*
(or hyper-threads specifically on Intel hardware), that each can follow one
sequence of instructions.  Then, the hardware threads are CPUs from the CPU idle
time management perspective and if the processor is asked to enter an idle state
by one of them, the hardware thread (or CPU) that asked for it is stopped, but
nothing more happens, unless all of the other hardware threads within the same
core also have asked the processor to enter an idle state.  In that situation,
the core may be put into an idle state individually or a larger unit containing
it may be put into an idle state as a whole (if the other cores within the
larger unit are in idle states already).

Idle CPUs
---------

Logical CPUs, simply referred to as "CPUs" in what follows, are regarded as
*idle* by the Linux kernel when there are no tasks to run on them except for the
special "idle" task.

Tasks are the CPU scheduler's representation of work.  Each task consists of a
sequence of instructions to execute, or code, data to be manipulated while
running that code, and some context information that needs to be loaded into the
processor every time the task's code is run by a CPU.  The CPU scheduler
distributes work by assigning tasks to run to the CPUs present in the system.

Tasks can be in various states.  In particular, they are *runnable* if there are
no specific conditions preventing their code from being run by a CPU as long as
there is a CPU available for that (for example, they are not waiting for any
events to occur or similar).  When a task becomes runnable, the CPU scheduler
assigns it to one of the available CPUs to run and if there are no more runnable
tasks assigned to it, the CPU will load the given task's context and run its
code (from the instruction following the last one executed so far, possibly by
another CPU).  [If there are multiple runnable tasks assigned to one CPU
simultaneously, they will be subject to prioritization and time sharing in order
to allow them to make some progress over time.]

The special "idle" task becomes runnable if there are no other runnable tasks
assigned to the given CPU and the CPU is then regarded as idle.  In other words,
in Linux idle CPUs run the code of the "idle" task called *the idle loop*.  That
code may cause the processor to be put into one of its idle states, if they are
supported, in order to save energy, but if the processor does not support any
idle states, or there is not enough time to spend in an idle state before the
next wakeup event, or there are strict latency constraints preventing any of the
available idle states from being used, the CPU will simply execute more or less
useless instructions in a loop until it is assigned a new task to run.


.. _idle-loop:

The Idle Loop
=============

The idle loop code takes two major steps in every iteration of it.  First, it
calls into a code module referred to as the *governor* that belongs to the CPU
idle time management subsystem called ``CPUIdle`` to select an idle state for
the CPU to ask the hardware to enter.  Second, it invokes another code module
from the ``CPUIdle`` subsystem, called the *driver*, to actually ask the
processor hardware to enter the idle state selected by the governor.

The role of the governor is to find an idle state most suitable for the
conditions at hand.  For this purpose, idle states that the hardware can be
asked to enter by logical CPUs are represented in an abstract way independent of
the platform or the processor architecture and organized in a one-dimensional
(linear) array.  That array has to be prepared and supplied by the ``CPUIdle``
driver matching the platform the kernel is running on at the initialization
time.  This allows ``CPUIdle`` governors to be independent of the underlying
hardware and to work with any platforms that the Linux kernel can run on.

Each idle state present in that array is characterized by two parameters to be
taken into account by the governor, the *target residency* and the (worst-case)
*exit latency*.  The target residency is the minimum time the hardware must
spend in the given state, including the time needed to enter it (which may be
substantial), in order to save more energy than it would save by entering one of
the shallower idle states instead.  [The "depth" of an idle state roughly
corresponds to the power drawn by the processor in that state.]  The exit
latency, in turn, is the maximum time it will take a CPU asking the processor
hardware to enter an idle state to start executing the first instruction after a
wakeup from that state.  Note that in general the exit latency also must cover
the time needed to enter the given state in case the wakeup occurs when the
hardware is entering it and it must be entered completely to be exited in an
ordered manner.

There are two types of information that can influence the governor's decisions.
First of all, the governor knows the time until the closest timer event.  That
time is known exactly, because the kernel programs timers and it knows exactly
when they will trigger, and it is the maximum time the hardware that the given
CPU depends on can spend in an idle state, including the time necessary to enter
and exit it.  However, the CPU may be woken up by a non-timer event at any time
(in particular, before the closest timer triggers) and it generally is not known
when that may happen.  The governor can only see how much time the CPU actually
was idle after it has been woken up (that time will be referred to as the *idle
duration* from now on) and it can use that information somehow along with the
time until the closest timer to estimate the idle duration in future.  How the
governor uses that information depends on what algorithm is implemented by it
and that is the primary reason for having more than one governor in the
``CPUIdle`` subsystem.

There are four ``CPUIdle`` governors available, ``menu``, `TEO <teo-gov_>`_,
``ladder`` and ``haltpoll``.  Which of them is used by default depends on the
configuration of the kernel and in particular on whether or not the scheduler
tick can be `stopped by the idle loop <idle-cpus-and-tick_>`_.  Available
governors can be read from the :file:`available_governors`, and the governor
can be changed at runtime.  The name of the ``CPUIdle`` governor currently
used by the kernel can be read from the :file:`current_governor_ro` or
:file:`current_governor` file under :file:`/sys/devices/system/cpu/cpuidle/`
in ``sysfs``.

Which ``CPUIdle`` driver is used, on the other hand, usually depends on the
platform the kernel is running on, but there are platforms with more than one
matching driver.  For example, there are two drivers that can work with the
majority of Intel platforms, ``intel_idle`` and ``acpi_idle``, one with
hardcoded idle states information and the other able to read that information
from the system's ACPI tables, respectively.  Still, even in those cases, the
driver chosen at the system initialization time cannot be replaced later, so the
decision on which one of them to use has to be made early (on Intel platforms
the ``acpi_idle`` driver will be used if ``intel_idle`` is disabled for some
reason or if it does not recognize the processor).  The name of the ``CPUIdle``
driver currently used by the kernel can be read from the :file:`current_driver`
file under :file:`/sys/devices/system/cpu/cpuidle/` in ``sysfs``.


.. _idle-cpus-and-tick:

Idle CPUs and The Scheduler Tick
================================

The scheduler tick is a timer that triggers periodically in order to implement
the time sharing strategy of the CPU scheduler.  Of course, if there are
multiple runnable tasks assigned to one CPU at the same time, the only way to
allow them to make reasonable progress in a given time frame is to make them
share the available CPU time.  Namely, in rough approximation, each task is
given a slice of the CPU time to run its code, subject to the scheduling class,
prioritization and so on and when that time slice is used up, the CPU should be
switched over to running (the code of) another task.  The currently running task
may not want to give the CPU away voluntarily, however, and the scheduler tick
is there to make the switch happen regardless.  That is not the only role of the
tick, but it is the primary reason for using it.

The scheduler tick is problematic from the CPU idle time management perspective,
because it triggers periodically and relatively often (depending on the kernel
configuration, the length of the tick period is between 1 ms and 10 ms).
Thus, if the tick is allowed to trigger on idle CPUs, it will not make sense
for them to ask the hardware to enter idle states with target residencies above
the tick period length.  Moreover, in that case the idle duration of any CPU
will never exceed the tick period length and the energy used for entering and
exiting idle states due to the tick wakeups on idle CPUs will be wasted.

Fortunately, it is not really necessary to allow the tick to trigger on idle
CPUs, because (by definition) they have no tasks to run except for the special
"idle" one.  In other words, from the CPU scheduler perspective, the only user
of the CPU time on them is the idle loop.  Since the time of an idle CPU need
not be shared between multiple runnable tasks, the primary reason for using the
tick goes away if the given CPU is idle.  Consequently, it is possible to stop
the scheduler tick entirely on idle CPUs in principle, even though that may not
always be worth the effort.

Whether or not it makes sense to stop the scheduler tick in the idle loop
depends on what is expected by the governor.  First, if there is another
(non-tick) timer due to trigger within the tick range, stopping the tick clearly
would be a waste of time, even though the timer hardware may not need to be
reprogrammed in that case.  Second, if the governor is expecting a non-timer
wakeup within the tick range, stopping the tick is not necessary and it may even
be harmful.  Namely, in that case the governor will select an idle state with
the target residency within the time until the expected wakeup, so that state is
going to be relatively shallow.  The governor really cannot select a deep idle
state then, as that would contradict its own expectation of a wakeup in short
order.  Now, if the wakeup really occurs shortly, stopping the tick would be a
waste of time and in this case the timer hardware would need to be reprogrammed,
which is expensive.  On the other hand, if the tick is stopped and the wakeup
does not occur any time soon, the hardware may spend indefinite amount of time
in the shallow idle state selected by the governor, which will be a waste of
energy.  Hence, if the governor is expecting a wakeup of any kind within the
tick range, it is better to allow the tick trigger.  Otherwise, however, the
governor will select a relatively deep idle state, so the tick should be stopped
so that it does not wake up the CPU too early.

In any case, the governor knows what it is expecting and the decision on whether
or not to stop the scheduler tick belongs to it.  Still, if the tick has been
stopped already (in one of the previous iterations of the loop), it is better
to leave it as is and the governor needs to take that into account.

The kernel can be configured to disable stopping the scheduler tick in the idle
loop altogether.  That can be done through the build-time configuration of it
(by unsetting the ``CONFIG_NO_HZ_IDLE`` configuration option) or by passing
``nohz=off`` to it in the command line.  In both cases, as the stopping of the
scheduler tick is disabled, the governor's decisions regarding it are simply
ignored by the idle loop code and the tick is never stopped.

The systems that run kernels configured to allow the scheduler tick to be
stopped on idle CPUs are referred to as *tickless* systems and they are
generally regarded as more energy-efficient than the systems running kernels in
which the tick cannot be stopped.  If the given system is tickless, it will use
the ``menu`` governor by default and if it is not tickless, the default
``CPUIdle`` governor on it will be ``ladder``.


.. _menu-gov:

The ``menu`` Governor
=====================

The ``menu`` governor is the default ``CPUIdle`` governor for tickless systems.
It is quite complex, but the basic principle of its design is straightforward.
Namely, when invoked to select an idle state for a CPU (i.e. an idle state that
the CPU will ask the processor hardware to enter), it attempts to predict the
idle duration and uses the predicted value for idle state selection.

It first uses a simple pattern recognition algorithm to obtain a preliminary
idle duration prediction.  Namely, it saves the last 8 observed idle duration
values and, when predicting the idle duration next time, it computes the average
and variance of them.  If the variance is small (smaller than 400 square
milliseconds) or it is small relative to the average (the average is greater
that 6 times the standard deviation), the average is regarded as the "typical
interval" value.  Otherwise, either the longest or the shortest (depending on
which one is farther from the average) of the saved observed idle duration
values is discarded and the computation is repeated for the remaining ones.

Again, if the variance of them is small (in the above sense), the average is
taken as the "typical interval" value and so on, until either the "typical
interval" is determined or too many data points are disregarded.  In the latter
case, if the size of the set of data points still under consideration is
sufficiently large, the next idle duration is not likely to be above the largest
idle duration value still in that set, so that value is taken as the predicted
next idle duration.  Finally, if the set of data points still under
consideration is too small, no prediction is made.

If the preliminary prediction of the next idle duration computed this way is
long enough, the governor obtains the time until the closest timer event with
the assumption that the scheduler tick will be stopped.  That time, referred to
as the *sleep length* in what follows, is the upper bound on the time before the
next CPU wakeup.  It is used to determine the sleep length range, which in turn
is needed to get the sleep length correction factor.

The ``menu`` governor maintains an array containing several correction factor
values that correspond to different sleep length ranges organized so that each
range represented in the array is approximately 10 times wider than the previous
one.

The correction factor for the given sleep length range (determined before
selecting the idle state for the CPU) is updated after the CPU has been woken
up and the closer the sleep length is to the observed idle duration, the closer
to 1 the correction factor becomes (it must fall between 0 and 1 inclusive).
The sleep length is multiplied by the correction factor for the range that it
falls into to obtain an approximation of the predicted idle duration that is
compared to the "typical interval" determined previously and the minimum of
the two is taken as the final idle duration prediction.

If the "typical interval" value is small, which means that the CPU is likely
to be woken up soon enough, the sleep length computation is skipped as it may
be costly and the idle duration is simply predicted to equal the "typical
interval" value.

Now, the governor is ready to walk the list of idle states and choose one of
them.  For this purpose, it compares the target residency of each state with
the predicted idle duration and the exit latency of it with the with the latency
limit coming from the power management quality of service, or `PM QoS <cpu-pm-qos_>`_,
framework.  It selects the state with the target residency closest to the predicted
idle duration, but still below it, and exit latency that does not exceed the
limit.

In the final step the governor may still need to refine the idle state selection
if it has not decided to `stop the scheduler tick <idle-cpus-and-tick_>`_.  That
happens if the idle duration predicted by it is less than the tick period and
the tick has not been stopped already (in a previous iteration of the idle
loop).  Then, the sleep length used in the previous computations may not reflect
the real time until the closest timer event and if it really is greater than
that time, the governor may need to select a shallower state with a suitable
target residency.


.. _teo-gov:

The Timer Events Oriented (TEO) Governor
========================================

The timer events oriented (TEO) governor is an alternative ``CPUIdle`` governor
for tickless systems.  It follows the same basic strategy as the ``menu`` `one
<menu-gov_>`_: it always tries to find the deepest idle state suitable for the
given conditions.  However, it applies a different approach to that problem.

.. kernel-doc:: drivers/cpuidle/governors/teo.c
   :doc: teo-description

.. _idle-states-representation:

Representation of Idle States
=============================

For the CPU idle time management purposes all of the physical idle states
supported by the processor have to be represented as a one-dimensional array of
|struct cpuidle_state| objects each allowing an individual (logical) CPU to ask
the processor hardware to enter an idle state of certain properties.  If there
is a hierarchy of units in the processor, one |struct cpuidle_state| object can
cover a combination of idle states supported by the units at different levels of
the hierarchy.  In that case, the `target residency and exit latency parameters
of it <idle-loop_>`_, must reflect the properties of the idle state at the
deepest level (i.e. the idle state of the unit containing all of the other
units).

For example, take a processor with two cores in a larger unit referred to as
a "module" and suppose that asking the hardware to enter a specific idle state
(say "X") at the "core" level by one core will trigger the module to try to
enter a specific idle state of its own (say "MX") if the other core is in idle
state "X" already.  In other words, asking for idle state "X" at the "core"
level gives the hardware a license to go as deep as to idle state "MX" at the
"module" level, but there is no guarantee that this is going to happen (the core
asking for idle state "X" may just end up in that state by itself instead).
Then, the target residency of the |struct cpuidle_state| object representing
idle state "X" must reflect the minimum time to spend in idle state "MX" of
the module (including the time needed to enter it), because that is the minimum
time the CPU needs to be idle to save any energy in case the hardware enters
that state.  Analogously, the exit latency parameter of that object must cover
the exit time of idle state "MX" of the module (and usually its entry time too),
because that is the maximum delay between a wakeup signal and the time the CPU
will start to execute the first new instruction (assuming that both cores in the
module will always be ready to execute instructions as soon as the module
becomes operational as a whole).

There are processors without direct coordination between different levels of the
hierarchy of units inside them, however.  In those cases asking for an idle
state at the "core" level does not automatically affect the "module" level, for
example, in any way and the ``CPUIdle`` driver is responsible for the entire
handling of the hierarchy.  Then, the definition of the idle state objects is
entirely up to the driver, but still the physical properties of the idle state
that the processor hardware finally goes into must always follow the parameters
used by the governor for idle state selection (for instance, the actual exit
latency of that idle state must not exceed the exit latency parameter of the
idle state object selected by the governor).

In addition to the target residency and exit latency idle state parameters
discussed above, the objects representing idle states each contain a few other
parameters describing the idle state and a pointer to the function to run in
order to ask the hardware to enter that state.  Also, for each
|struct cpuidle_state| object, there is a corresponding
:c:type:`struct cpuidle_state_usage <cpuidle_state_usage>` one containing usage
statistics of the given idle state.  That information is exposed by the kernel
via ``sysfs``.

For each CPU in the system, there is a :file:`/sys/devices/system/cpu/cpu<N>/cpuidle/`
directory in ``sysfs``, where the number ``<N>`` is assigned to the given
CPU at the initialization time.  That directory contains a set of subdirectories
called :file:`state0`, :file:`state1` and so on, up to the number of idle state
objects defined for the given CPU minus one.  Each of these directories
corresponds to one idle state object and the larger the number in its name, the
deeper the (effective) idle state represented by it.  Each of them contains
a number of files (attributes) representing the properties of the idle state
object corresponding to it, as follows:

``above``
	Total number of times this idle state had been asked for, but the
	observed idle duration was certainly too short to match its target
	residency.

``below``
	Total number of times this idle state had been asked for, but certainly
	a deeper idle state would have been a better match for the observed idle
	duration.

``desc``
	Description of the idle state.

``disable``
	Whether or not this idle state is disabled.

``default_status``
	The default status of this state, "enabled" or "disabled".

``latency``
	Exit latency of the idle state in microseconds.

``name``
	Name of the idle state.

``power``
	Power drawn by hardware in this idle state in milliwatts (if specified,
	0 otherwise).

``residency``
	Target residency of the idle state in microseconds.

``time``
	Total time spent in this idle state by the given CPU (as measured by the
	kernel) in microseconds.

``usage``
	Total number of times the hardware has been asked by the given CPU to
	enter this idle state.

``rejected``
	Total number of times a request to enter this idle state on the given
	CPU was rejected.

The :file:`desc` and :file:`name` files both contain strings.  The difference
between them is that the name is expected to be more concise, while the
description may be longer and it may contain white space or special characters.
The other files listed above contain integer numbers.

The :file:`disable` attribute is the only writeable one.  If it contains 1, the
given idle state is disabled for this particular CPU, which means that the
governor will never select it for this particular CPU and the ``CPUIdle``
driver will never ask the hardware to enter it for that CPU as a result.
However, disabling an idle state for one CPU does not prevent it from being
asked for by the other CPUs, so it must be disabled for all of them in order to
never be asked for by any of them.  [Note that, due to the way the ``ladder``
governor is implemented, disabling an idle state prevents that governor from
selecting any idle states deeper than the disabled one too.]

If the :file:`disable` attribute contains 0, the given idle state is enabled for
this particular CPU, but it still may be disabled for some or all of the other
CPUs in the system at the same time.  Writing 1 to it causes the idle state to
be disabled for this particular CPU and writing 0 to it allows the governor to
take it into consideration for the given CPU and the driver to ask for it,
unless that state was disabled globally in the driver (in which case it cannot
be used at all).

The :file:`power` attribute is not defined very well, especially for idle state
objects representing combinations of idle states at different levels of the
hierarchy of units in the processor, and it generally is hard to obtain idle
state power numbers for complex hardware, so :file:`power` often contains 0 (not
available) and if it contains a nonzero number, that number may not be very
accurate and it should not be relied on for anything meaningful.

The number in the :file:`time` file generally may be greater than the total time
really spent by the given CPU in the given idle state, because it is measured by
the kernel and it may not cover the cases in which the hardware refused to enter
this idle state and entered a shallower one instead of it (or even it did not
enter any idle state at all).  The kernel can only measure the time span between
asking the hardware to enter an idle state and the subsequent wakeup of the CPU
and it cannot say what really happened in the meantime at the hardware level.
Moreover, if the idle state object in question represents a combination of idle
states at different levels of the hierarchy of units in the processor,
the kernel can never say how deep the hardware went down the hierarchy in any
particular case.  For these reasons, the only reliable way to find out how
much time has been spent by the hardware in different idle states supported by
it is to use idle state residency counters in the hardware, if available.

Generally, an interrupt received when trying to enter an idle state causes the
idle state entry request to be rejected, in which case the ``CPUIdle`` driver
may return an error code to indicate that this was the case. The :file:`usage`
and :file:`rejected` files report the number of times the given idle state
was entered successfully or rejected, respectively.

.. _cpu-pm-qos:

Power Management Quality of Service for CPUs
============================================

The power management quality of service (PM QoS) framework in the Linux kernel
allows kernel code and user space processes to set constraints on various
energy-efficiency features of the kernel to prevent performance from dropping
below a required level.

CPU idle time management can be affected by PM QoS in two ways, through the
global CPU latency limit and through the resume latency constraints for
individual CPUs.  Kernel code (e.g. device drivers) can set both of them with
the help of special internal interfaces provided by the PM QoS framework.  User
space can modify the former by opening the :file:`cpu_dma_latency` special
device file under :file:`/dev/` and writing a binary value (interpreted as a
signed 32-bit integer) to it.  In turn, the resume latency constraint for a CPU
can be modified from user space by writing a string (representing a signed
32-bit integer) to the :file:`power/pm_qos_resume_latency_us` file under
:file:`/sys/devices/system/cpu/cpu<N>/` in ``sysfs``, where the CPU number
``<N>`` is allocated at the system initialization time.  Negative values
will be rejected in both cases and, also in both cases, the written integer
number will be interpreted as a requested PM QoS constraint in microseconds.

The requested value is not automatically applied as a new constraint, however,
as it may be less restrictive (greater in this particular case) than another
constraint previously requested by someone else.  For this reason, the PM QoS
framework maintains a list of requests that have been made so far for the
global CPU latency limit and for each individual CPU, aggregates them and
applies the effective (minimum in this particular case) value as the new
constraint.

In fact, opening the :file:`cpu_dma_latency` special device file causes a new
PM QoS request to be created and added to a global priority list of CPU latency
limit requests and the file descriptor coming from the "open" operation
represents that request.  If that file descriptor is then used for writing, the
number written to it will be associated with the PM QoS request represented by
it as a new requested limit value.  Next, the priority list mechanism will be
used to determine the new effective value of the entire list of requests and
that effective value will be set as a new CPU latency limit.  Thus requesting a
new limit value will only change the real limit if the effective "list" value is
affected by it, which is the case if it is the minimum of the requested values
in the list.

The process holding a file descriptor obtained by opening the
:file:`cpu_dma_latency` special device file controls the PM QoS request
associated with that file descriptor, but it controls this particular PM QoS
request only.

Closing the :file:`cpu_dma_latency` special device file or, more precisely, the
file descriptor obtained while opening it, causes the PM QoS request associated
with that file descriptor to be removed from the global priority list of CPU
latency limit requests and destroyed.  If that happens, the priority list
mechanism will be used again, to determine the new effective value for the whole
list and that value will become the new limit.

In turn, for each CPU there is one resume latency PM QoS request associated with
the :file:`power/pm_qos_resume_latency_us` file under
:file:`/sys/devices/system/cpu/cpu<N>/` in ``sysfs`` and writing to it causes
this single PM QoS request to be updated regardless of which user space
process does that.  In other words, this PM QoS request is shared by the entire
user space, so access to the file associated with it needs to be arbitrated
to avoid confusion.  [Arguably, the only legitimate use of this mechanism in
practice is to pin a process to the CPU in question and let it use the
``sysfs`` interface to control the resume latency constraint for it.]  It is
still only a request, however.  It is an entry in a priority list used to
determine the effective value to be set as the resume latency constraint for the
CPU in question every time the list of requests is updated this way or another
(there may be other requests coming from kernel code in that list).

CPU idle time governors are expected to regard the minimum of the global
(effective) CPU latency limit and the effective resume latency constraint for
the given CPU as the upper limit for the exit latency of the idle states that
they are allowed to select for that CPU.  They should never select any idle
states with exit latency beyond that limit.

While the above CPU QoS constraints apply to CPU idle time management, user
space may also request a CPU system wakeup latency QoS limit, via the
`cpu_wakeup_latency` file.  This QoS constraint is respected when selecting a
suitable idle state for the CPUs, while entering the system-wide suspend-to-idle
sleep state, but also to the regular CPU idle time management.

Note that, the management of the `cpu_wakeup_latency` file works according to
the 'cpu_dma_latency' file from user space point of view.  Moreover, the unit
is also microseconds.

Idle States Control Via Kernel Command Line
===========================================

In addition to the ``sysfs`` interface allowing individual idle states to be
`disabled for individual CPUs <idle-states-representation_>`_, there are kernel
command line parameters affecting CPU idle time management.

The ``cpuidle.off=1`` kernel command line option can be used to disable the
CPU idle time management entirely.  It does not prevent the idle loop from
running on idle CPUs, but it prevents the CPU idle time governors and drivers
from being invoked.  If it is added to the kernel command line, the idle loop
will ask the hardware to enter idle states on idle CPUs via the CPU architecture
support code that is expected to provide a default mechanism for this purpose.
That default mechanism usually is the least common denominator for all of the
processors implementing the architecture (i.e. CPU instruction set) in question,
however, so it is rather crude and not very energy-efficient.  For this reason,
it is not recommended for production use.

The ``cpuidle.governor=`` kernel command line switch allows the ``CPUIdle``
governor to use to be specified.  It has to be appended with a string matching
the name of an available governor (e.g. ``cpuidle.governor=menu``) and that
governor will be used instead of the default one.  It is possible to force
the ``menu`` governor to be used on the systems that use the ``ladder`` governor
by default this way, for example.

The other kernel command line parameters controlling CPU idle time management
described below are only relevant for the *x86* architecture and references
to ``intel_idle`` affect Intel processors only.

The *x86* architecture support code recognizes three kernel command line
options related to CPU idle time management: ``idle=poll``, ``idle=halt``,
and ``idle=nomwait``.  The first two of them disable the ``acpi_idle`` and
``intel_idle`` drivers altogether, which effectively causes the entire
``CPUIdle`` subsystem to be disabled and makes the idle loop invoke the
architecture support code to deal with idle CPUs.  How it does that depends on
which of the two parameters is added to the kernel command line.  In the
``idle=halt`` case, the architecture support code will use the ``HLT``
instruction of the CPUs (which, as a rule, suspends the execution of the program
and causes the hardware to attempt to enter the shallowest available idle state)
for this purpose, and if ``idle=poll`` is used, idle CPUs will execute a
more or less "lightweight" sequence of instructions in a tight loop.  [Note
that using ``idle=poll`` is somewhat drastic in many cases, as preventing idle
CPUs from saving almost any energy at all may not be the only effect of it.
For example, on Intel hardware it effectively prevents CPUs from using
P-states (see |cpufreq|) that require any number of CPUs in a package to be
idle, so it very well may hurt single-thread computations performance as well as
energy-efficiency.  Thus using it for performance reasons may not be a good idea
at all.]

The ``idle=nomwait`` option prevents the use of ``MWAIT`` instruction of
the CPU to enter idle states. When this option is used, the ``acpi_idle``
driver will use the ``HLT`` instruction instead of ``MWAIT``. On systems
running Intel processors, this option disables the ``intel_idle`` driver
and forces the use of the ``acpi_idle`` driver instead. Note that in either
case, ``acpi_idle`` driver will function only if all the information needed
by it is in the system's ACPI tables.

In addition to the architecture-level kernel command line options affecting CPU
idle time management, there are parameters affecting individual ``CPUIdle``
drivers that can be passed to them via the kernel command line.  Specifically,
the ``intel_idle.max_cstate=<n>`` and ``processor.max_cstate=<n>`` parameters,
where ``<n>`` is an idle state index also used in the name of the given
state's directory in ``sysfs`` (see
`Representation of Idle States <idle-states-representation_>`_), causes the
``intel_idle`` and ``acpi_idle`` drivers, respectively, to discard all of the
idle states deeper than idle state ``<n>``.  In that case, they will never ask
for any of those idle states or expose them to the governor.  [The behavior of
the two drivers is different for ``<n>`` equal to ``0``.  Adding
``intel_idle.max_cstate=0`` to the kernel command line disables the
``intel_idle`` driver and allows ``acpi_idle`` to be used, whereas
``processor.max_cstate=0`` is equivalent to ``processor.max_cstate=1``.
Also, the ``acpi_idle`` driver is part of the ``processor`` kernel module that
can be loaded separately and ``max_cstate=<n>`` can be passed to it as a module
parameter when it is loaded.]